Nowadays, the use of agricultural by‐products, as the cheap substrate for the production of value‐added products, is of high interest for the researchers and practitioners. Cellulase is a relatively expensive and a very important industrial enzyme where in this study was produced form rice by‐products under solid‐state fermentation. A new mutant of Trichoderma reesei was used for cellulase production. The effective variables were initially screened by “Plackett and Burman design.” Afterward, the main variables including moisture content, P source, incubation temperature, and incubation time were optimized by “one factor at a time design.” Finally, the resulting variables including 74% for moisture content, 2 g/L for K₂HPO₄, 30°C of incubation temperature, and 4 days of incubation time were reported as the ultimate optimal condition for cellulase production.